Hi @selenesolveig
This is because of data sanitization,
If you really want to add a link we can provide you some steps. You have to remove the data sanitization function which we don’t recommend.
Step 1) Go to root folder of theme ( wp-content/themes/elvinaa)
Step 2) Go to inc/customizer/customizer.php and open this file in any text editor
Step 3) Find line number 984 and remove the sanitization_callback function ie line should be like
'sanitize_callback' => ''
Step 4) Now go to inc/template-functions.php and open this in text editor
Step 5) Find line no 186 and remove the esc_attr() function. The line should look like
<p><?php echo get_theme_mod( 'el_copyright_text', __('Copyrights Elvinaa. All Rights Reserved','elvinaa') ); ?><span><?php _e(' | Theme by ','elvinaa') ?><a href="https://www.spiraclethemes.com/" target="_blank"><?php _e('Spiraclethemes','elvinaa') ?></a></span></p>
Step 6) save both files and try again to add a link
Note: Do not update the theme as you will lose these changes after the update.
Regards